Ultrasonic motors in which the ultrasonic actuator comprises friction elements disposed on or fixed to the latter, for example, are known from publications U.S. Pat. Nos. 6,979,934 B1, 6,765,335 B2 and EP 2 153 476. In the case of these ultrasonic motors, the ultrasonic actuator is made completely of a lead-zirconate-titanate (PZT) ceramic. The friction elements are connected to the polarized ultrasonic actuator by means of an organic adhesive on an epoxy resin base. Furthermore, ultrasonic actuators which disclose the use of monocrystals as a material for the friction contact are known from publications DE 10 2008 026 429 A1 and DE 195 22 072 C1, whilst the use of monocrystals as a material for the oscillator is known from publications DE 199 26 010 A1 and JP 11261127 A.
The maximum operating temperature of the ultrasonic actuators is limited by the Curie point of the piezoceramic, which lies at over 300° C. for PZT ceramics. At the same time, the strength of the organic adhesive on an epoxy resin base is determined by the setting temperature of the adhesive. In the case of special adhesives, this can amount to up to 250° C.
In the ultrasonic motor, the friction element transmits the mechanical power of the ultrasonic actuator to the element to be driven. The total mechanical power to be transmitted to the element to be driven is therefore transmitted via the adhesive layer that connects the friction element to the ultrasonic actuator. As a result of the ultrasonic oscillations acting on the adhesive layer, the latter heats up.
If the temperature of the adhesive layer exceeds the setting temperature of the adhesive, this usually leads to a decomposition of the adhesive with accompanying serious changes in the properties, so that the friction element subsequently becomes detached from the ultrasonic actuator. The ultrasonic actuator thus becomes load-free, as a result of which its resistance is reduced by several powers of ten. This leads to destruction of the ultrasonic actuator and ultimately to a total failure of the ultrasonic motor.
The temperature increase in the adhesive layer is proportional to its thickness. It is therefore advantageous if the thickness of the adhesive layer is small. Thicknesses of the adhesive layer between 10 and 15 μm are common.
According to the applicant's in-house prior art, when the friction element is bonded with the piezoelectric actuator of the ultrasonic motor, both are heated up to the setting temperature of the adhesive. Cooling of the two elements then takes place. On account of large differences in the temperature expansion coefficients of the ultrasonic actuator and the friction element, large initial mechanical stresses are created in the region of the adhesive layer during cooling. These initial mechanical stresses are reduced or compensated for by the temperature increase usually arising during the operation of the ultrasonic actuator; the farther away the setting temperature of the adhesive lies from the average operating temperature of the motor, however, the higher the mechanical stresses.
In practice, it is almost impossible to select the setting temperature of the adhesive higher than 100° C. The initial mechanical stresses would be extremely high at a higher temperature. When the motor is switched on, the initial mechanical stresses and the mechanical stresses add up on account of the friction contact of the friction element and the element to be driven, as a result of which cumulative mechanical stresses would arise, the amplitude whereof would be so great that the piezoelement would explode or the adhesive layer would be directly destroyed.
On the other hand, it is also not practicable to increase the thickness of the adhesive layer to compensate for the difference in the temperature expansion of the piezoceramic and the friction element, since—as mentioned previously—the heating of the adhesive layer is proportional to its thickness.
The maximum operating temperature is therefore very restricted or limited with the ultrasonic actuators known from the applicant's in-house prior art and the associated ultrasonic motors.